Initial Corrosion Behaviors of Fe20Cr25NiNb Stainless Steel in High Temperature Environment with Different Relative Humidity Values

The initial corrosion behaviors of Fe20Cr25NiNb austenitic stainless steel are investigated at 1000 degrees C in helium environment with different relative humidity values. Results show that thin chromia film and small nodules are formed for all oxidation conditions, and some white-colored large nodules are developed for the conditions with >= 20% relative humidity. The number and size of these large nodules increase slightly with the increase of relative humidity. These small nodules are primarily composed of the pre-formed chromia film and new-generated chromia phases, and their formation is strongly related to the oxidation of intergranular Fe2Nb phases. The white-colored large nodules mainly consist of three oxide layers: outer magnetite layer, inner spinel layer and (Cr, O)-rich healing layer. An accelerated oxidation is observed after the transition point for the conditions with >= 20% relative humidity, which is attributed to the formation of large nodules. The oxide growth primarily follows the linear law before the transition point and obeys the parabolic law after the transition point for higher relative humidity.